use some abs or similar plastic, 1/4" (ideally 1/2"-3/4" or more, for better protection), you can get it in black.
cut it so you have 1/2" reveal all the way around, when centered under the post.
it will create a simple shadow line, and help prevent water from being drawn up into the post.
and you can add some ice and water shield to the post bottoms, or heavily coat them in anchor seal.
you do need to get them up off the deck.
